1. Field of the Invention
This invention relates to a safety lock for knob-operated doors, and in particular, those devices using a cable or chain positioned over a doorknob to prevent the door from being pushed open.
2. Prior Art Devices
There presently:exists a number of devices and methods for securing knob-operated doors. Among these are: U.S. Pat. No. 4,478,442 by Martin; U.S. Pat. No. 4,135,748 by Correnti et al; U.S. Pat. No. 4,155,577 by Raymond; U.S. Pat. No. 4,135,748 by Roberts; U.S. Pat. No. 3,640,558 by Gewirtz et al; U.S. Pat. No. 3,451,708 by Brooks, and U.S. Pat. No. 878,607 by Cairns. The last cited patent by Cairns, was issued in 1908, indicating that the concept of restraining knob-operated doors by chain or cable is quite old.
With the exception of Martin and Correnti, all of the above cited inventions utilize chains which fasten around the doorknob on the inside of the door and also in some manner are attached to the doorway framework. Thus any knob-operated door may be prevented from being fully opened.
Martin utilizes an anchor block fastened by a lag bolt to a doorway framework, a wire cable which is wrapped around a doorknob and is fastened to the anchor block, and a sleeve which fits over the cable.
Correnti et al utilizes a box to be placed in the wall of a doorway for anchoring a cable; a cable which is wrapped around a doorknob and a special two-section sleeve which fits over the cable between it and the doorknob. Provision is made for storing the cable and sleeve in the wall box when the device is not in use.
A review of the Martin device shows that the cable security depends on beads or slugs which are attached to its ends and prevent the cable from being pulled through holes in the anchor block. Experience with similarly beaded cable ends has shown that such beads or slugs may be stripped off if sufficient tension force is applied, as could happen if someone tried to force a door restrained by a cable over a doorknob. In other cable door lock devices, the cable ends may be butted and clamped. However, this method also is considered to be relatively weak in securing a cable against a strong applied tension force, since the butted cable ends may be pulled apart.
The Correnti et al device, on the other hand, while incorporating strong restraint on its cable ends, requires a skilled installation of its wall box; something the average homeowner might not be able or wish to do.
In view of Martin and Correnti et al and similar devices, there is an apparent need for a cable locking device for knob operated doors which incorporates very strong restraint on its cable ends to resist forcing open, and is simple enough in construction for any average homeowner to install in a doorway.